1. Field of the Invention
The invention relates to underbed thread pull-off mechanisms for use in lockstitch sewing machines, and more particularly to bobbin thread pull-off mechanisms for such use which can be variably controlled to provide for the withdrawal of suitable amounts of thread from the bobbin of a lockstitch sewing machine depending upon the particular sewing operation to be performed.
2. Description of the Prior Art
It is well known to provide a lockstitch machine with means for pulling thread from a bobbin spool for use in the formation of stitches. Most commonly, such means have been adapted to supply a constant amount of thread for each stitch regardless of the length of stitch called for, the type of stitch to be formed, or the thickness of material being sewn. However, variable operable pull-off mechanisms with which the amount of thread withdrawn from a bobbin can be changed to meet a need defined by the type of operation to be performed on a sewing machine have been devised. U.S. Pat. No. 4,215,639 of Ralph E. Johnson for "Selectively Controllable Bobbin Thread Pull-Off Mechanism," issued Aug. 5, 1980, shows such a mechanism controllable as by a servomotor responsive to signals representing a selected stitch length, a presser bar position and needle bight. Abandoned U.S. patent application of Stanley J. Ketterer for "Variable Bobbin Thread Control for Lockstitch Looptaker," Ser. No. 055847, filed July 9, 1979, and assigned to the same assignee as the present application, discloses a shiftable pull-off and retraction element on a looptaker capable of providing an effect similar to the increased or decreased pull-off of bobbin thread. U.S. Pat. No. 4,182,250 of Ralph E. Johnson for "Bobbin Thread Control Means for a Lock Stitch Sewing Machine" issued Jan. 8, 1980, discloses mechanism for pulling thread from a bobbin spool in amounts which can be predetermined by the setting of a stitch length regulator.
Stitches which are neither too tight nor too loose are more readily formed with a variably controllable bobbin thread pull-off mechanism than with a pull-off mechanism which must always supply the same amount of bobbin thread for stitches. However, the known variably controllable bobbin thread pull-off mechanisms, if not structurally complex or somewhat insensitive to input signals, are at least limited in the signals to which they are adapted to respond.
It is a prime object of the present invention to provide an improved variably controllable bobbin thread pull-off mechanism which is lacking in complexity and which can be rendered readily responsive to multiple signals.
It is another object of the invention to provide an improved variably controllable bobbin thread pull-off mechanism as described which can be mounted on a shaft concentric with the rotational axis of a looptaker.
Other objects and advantages of the invention will become apparent during a reading of the specification taken in connection with the accompanying drawings.